January 9, 2020 – A weather system moves quickly through the area today leading to moderate travel impacts over the northern Sierra. Dry weather on Friday, then a couple of weather systems brush the region over the weekend.


The incoming weather system is moving into NorCal early this morning. Radar indicates precipitation over the Coast Range is beginning to spread east into the Sacramento Valley while light snow has been occurring overnight over the northern Sierra out ahead of the front. Current temperatures are running a little milder compared to 24 hours ago and range from the upper 20s and 30s in the mountains to the 40s elsewhere.

An active northwest flow aloft will allow for successive fast-moving wx systems into the weekend. Today’s will be a little wetter compared to the previous with a stronger vort max, better dynamics. QPF totals of around a quarter to a third of an inch are expected in the valley with around a half inch to three quarters of an inch over the northern mountains and northern Sierra. A broad area of 4 to 8 inch snowfall totals are expected over the mountains through this morning.

The system quickly exits this afternoon and clearing is expected from N to S into tonight. A strong temp inversion is forecast to develop by Fri morning and with plenty of moisture trapped below this inversion, there is likely to be stratus/fog/frost development in the mountain valleys/basins, and in the central valley. For the most part, Nly winds should inhibit development of such in the Nrn portion of the Vly, but can’t rule out some light frost in the wind sheltered areas. With temps dropping quickly after dark due to clearing conditions, and into the low to mid 30s in the Valley by morning, widespread frost is expected early Fri morning. Dewpoints may tend to be a little higher (above freezing) in the SJV area, where theoretically, the release of latent heat of condensation will keep mins milder and more conducive for the development of fog rather than frost.

Most of Friday looks dry until the Saturday system impacts the region. The forecasts have been tracking this system more to the east in our CWA, favoring the bulk of the QPF over the Sierra. Ensemble QPF forecasts show a similarity to the early week system; a quarter of an inch or so in the Sierra and a broad 2 to 4 inches of snow accumulation with a few hundredths in the Valley.

Extended Discussion (Monday through Thursday)

Longwave troughing will dominate the forecast over the extended periods leading to cool and wet weather. Several waves of precipitation are forecast to track over the region, with the first round arriving Sunday night into Monday. Preliminary QPF amounts indicate 0.50-1 inch of liquid precipitation over higher terrain, and up to 0.15 inches of Valley rain.

Snow levels will range between 2500 to 3500 feet, with 3-7 inches, locally higher, of snow currently expected for elevations above 3500 feet. As the shortwave exits the region conditions will start to dry out on Tuesday. The Valley will have the best chance of being dry while the mountains observe lingering showers.

Mid-late week, deterministic models continue to deviate from each other. It does look like a more vigorous system is on track to impact northern California. Timing, strength, and location very but the NBM hasn’t deviated from it’s path too much. Given this, there is still the potential for snow down into the lower Sierra foothills and the northern Valley floor. Specific storm details will likely change given this is still far out, so stay tuned to the forecast.

Regarding temperatures, a cold air mass originating from western Canada will bring much colder than normal temperatures next week. Temperatures are anticipated to be 5 to 15 degrees cooler than normal, with Valley highs in the upper 40s to low 50s by midweek.